1. Field of the Invention
The present invention relates to a latch for securing hinged cowlings surrounding an aircraft engine, and in particular, a latch with a preload indicator.
2. Description of Related Art
In the aircraft industry a preload latch applies a preload or predetermined load to a cowling structure to prevent or eliminate relative motion between the pair of hinged cowlings during aircraft operating conditions. It is known in the aircraft industry that an excessive amount of latch preload can cause damage to the cowling structure and/or the latch and that too little preload can cause excessive wear on mating cowlings due to relative motion.
During the initial design stage of the cowling, the latch loads are determined and included in the cowling installation procedures, rigging instructions and maintenance manuals. It is known that the force on the latch hook is directly related to the amount of force needed to rotate the latch handle to the closed position.
The method used to determine the correct preload has been to measure the maximum force to close the latch handle. The measurement is usually accomplished with a plunger type spring scale where the scale plunger is positioned to measure the force needed to move the handle to the closed position. The maximum force is noted, and if inadequate or excessive, the latch or keeper is adjusted and another measurement is taken. This is repeated until the proper preload is reached.
Prior to the subject invention, aircraft maintenance personnel, on occasion, would fail to properly adjust preload during routine checks, and sometimes during cowling rigging or installation, because a plunger-type spring scale was not readily available to measure the handle force. The correct latch preload, therefore, was sometimes not achieved.
Conventional latches for securing a pair of hinged cowlings surrounding an aircraft engine do not provide preload indication. Examples of such a conventional latch are disclosed in U.S. Pat. No. 4,531,769 to Glancy and U.S. Pat. No. 4,538,843 to Harris.
In the past, mechanisms using spring movement have been use to provide an indication of the force imposed in a particular connection, such as disclosed in U.S. Pat. No. 3,943,882 to Sollak et al. and Austrian Patent No. 243,658. These prior art latches, however, did not include a mechanism to indicate preload forces.
The improved latch is simple to design, rugged in construction and requires no additional parts over a typical latch since the preload indicator replaces a spring and accomplishes the same function as the replaced spring. Not only does the improve latch eliminate the need for maintenance personnel to provide a plunger type spring scale, but it also provides an additional safety feature during the life of the cowling in that the latch preload can be checked each time the cowling is closed and be adjusted at that time.
Advantages of the invention are set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.